Adding that extra day every four years keeps the calendar aligned with the Earth's revolutions around the sun.

It takes the Earth 365.24 days to make a complete cycle around the sun, meaning that every four years, the world would be a full day out of synch with the calendar without the additional day.

The Gregorian calendar decides which years are leap years by these criteria: the year can be evenly divided by four and 400, but cannot be evenly divided by 100.

Here are some notes, facts and history around leap day.

1. Leap year birthdays

Astrologers believe if you were born on leap day, you might have special abilities. If that's true, then these "leapers" are worth taking note of.

Pope Paul III: Born in 1468 and died in 1549. He was pope from October 1534 until his death in November 1549.

Former Montreal Canadiens player Henri Richard, front left, is one of the more notable people to be born on Feb. 29. (Paul Chiasson/The Canadian Press)

Henri Richard: Born in 1936, Richard was a hockey player for the Montreal Canadiens for 20 seasons. He holds the record for winning the most Stanley Cups as a National Hockey League player, with 11. Richard, known as the Pocket Rocket, is the younger brother of Hall of Famer Maurice (Rocket) Richard.

Khaled: Born in 1960, Khaled is an Algerian folk musician. He won a Grammy for his song Love to the People with the legendary Carlos Santana.

Mervyn Warren: Born in 1964, Warren is a five-time Grammy Award winner, as well as a 10-time nominee. He produced music across all genres and even tried his hand as a film composer.

Simon Gagne: Born in 1980, Gagne was a Canadian hockey player, primarily for the Philadelphia Flyers. He also played for the Los Angeles Kings, winning the Stanley Cup with them in 2012. After amassing 601 points in 822 career games, Gagne retired last year.

Cam Ward: Born in 1984, Ward is a goalie for the Carolina Hurricanes. The Alberta native wowed the hockey world with his play in his rookie year in 2006, leading the Hurricanes to a Stanley Cup win, and was named MVP of the playoffs.

Mark Foster: Born in 1984, Foster is a singer and frontman of the indie rock band, Foster the People. The band, founded by Foster, has been nominated for three Grammy Awards.

2. Historical events

Everything from natural disasters to Oscar wins and historic trade agreements have taken place on Feb. 29.

Pierre Trudeau announced he was stepping down as prime minister on Feb. 29, 1984, the day after he took a long walk in the snow. (Canadian Press)

Political events

1796: The Jay Treaty between the United States and Great Britain began, bringing 10 years of peaceful trade between the two nations.

1984: Canadian Prime Minister Pierre Elliott Trudeau announced he was resigning, after taking a walk in the snow the night before.

​Arts events

Hattie McDaniel, who acted in Gone with the Wind, was the first black person to win an Academy Award. (Associated Press)

1940: The Civil War drama, Gone with the Wind, won eight Oscars. It is considered by many as one of the best classic movies of all time.

1940: Hattie McDaniel, who played the role of Mammy in Gone with the Wind, won the Academy Award for best supporting actress, becoming the first black performer to win an Oscar.

2004: Lord of the Rings: The Return of the King won all 11 of the categories it was nominated for at the 76th Academy Awards. It is the eighth best-rated movie of all time by 1.1 million IMDb users.

Sports events

1972: Hank Aaron became the richest player in MLB history after signing a contract with the Atlanta Braves for $200,000. Compare that with Los Angeles Dodgers superstar Clayton Kershaw, who will earn $34.6 million in 2016, equivalent to $213,580 per team game.

1980: Gordie Howe became the first NHL player to score 800 career goals, a feat even more incredible considering that he was 51 years old. He remains second in all-time goals scored.

Disasters

1960: An earthquake killed one-third of population of Agadir, Morocco in just 15 seconds. More than 12,000 people died.

1996: 123 people were killed in an airplane crash 1.6 kilometres short of the runaway in Peru.

3. The Irish Feb. 29 legend

An old Irish legend says that on Feb. 29, women are allowed to propose to men. As it goes, if the man declines the proposal, he must pay the woman a fine, whether in a kiss, a pair of gloves or money for a silk dress.

Leap day is often referenced in TV shows, such as Parks and Recreation, Modern Family, The Middle and 30 Rock.

5. Struggles of being born on Feb. 29

For people born on the regular 365 days of the year, there may not be much thought about what happens to the nearly five million people worldwide who have leap day as their birthday.

Many of them struggle with day-to-day administrative hassles, such as setting up bank accounts or insurance policies, because some companies don't recognize Feb. 29 as a valid date. While technological improvements are making this less of an issue, some websites still won't accept Feb. 29.

Laura Pedersen has often had trouble with online application forms, insurance documents, Facebook and even her marriage licence.

But she says technology is getting much better and she doesn't have as many problems as she once did.

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